Electric utility companies utilize power line communication systems to provide a means for a central station to communicate command signals to remote receivers which are located at the sites of electric energy consumers. In some applications of power line communication systems, the remote receivers are configured as one-way devices which are able to receive commands from the central station and follow those commands which are, typically, instructions to shed a predetermined electrical load, such as a water heater or other nonessential devices, during periods of time when the electric utility experiences peak load demands. Power line communication systems can also be utilized in association with two-way remote devices which can also transmit information from the location of an electric energy consumer to the central station. These types of devices can be used to transmit electric energy consumption information or other types of data.
In either a one-way or a two-way power line communication system, a central station formulates a base band data message according to a preselected protocol. This message can be a command, intended to receipt by a remote receiver, to shed nonessential loads or to respond with a subsequent transmission containing data relating to information that is stored at the remote location. After the central station formulates the message in the form of base band data, the base band data bits are then used to modulate a high frequency signal. Although many different high frequency signals can be utilized in this type of system, a typical frequency is 12.5 kilohertz. The modulated signal is then imposed on a power line that is connected to both the central station and the remote receivers to which the message is directed. When a remote receiver detects the presence of an incoming message on the power line, it demodulates the message and examines the base band bits contained therein.
In a typical application, each remote receiver is provided with a unique address and, furthermore, groups of remote receivers are provided with block addresses. This permits the electric utility to address each receiver individually when required or, alternatively, to send a command message to an entire group of receivers simultaneously. When the central station wishes to command a remote receiver to read its own meter values and respond with a return transmission back to the central station containing this data, individual addressing is utilized. However, if the central station wishes to cause a group of remote receivers to shed their loads during a peak demand period, block addressing is used.
When a remote receiver detects that an incoming power line communication message is present on the power line, it detects the message and demodulates it. Once demodulated, the message can be examined by the remote receiver to determine whether or not the message contains its proper individual address or block address. In this way, the remote receiver can determine whether the message is addressed to it or to other remote receivers. If neither the block address nor the individual address contained in the message applies to the remote receiver, the message is ignored.
U.S. Pat. No. 4,065,763, which issued to Whyte et al. on Dec. 27, 1977, discloses a distribution network power line communication system and U.S. Pat. No. 4,142,178, which issued to Whyte et al. on Feb. 27, 1979, discloses a high voltage signal coupler that is used in a distribution network power line communication system. When power line communication signals are intended to travel a significant distance on the power line, repeaters are used to receive and retransmit the signals as they travel from the transmitter to the remote receiver. U.S. Pat. No. 4,032,911, which issued to Melvin on June 28, 1977, discloses a signal repeater for use in conjunction with a power line distribution system.
A typical application of a one-way remote receiver that is capable of responding to load shed commands is disclosed in U.S. Pat. No. 4,130,874, which issued on Dec. 19, 1978 to Pai and which describes a load management terminal having plural selectable address formats for a power line communication system. Also, U.S. Pat. No. 4,355,303, which issued on Oct. 19, 1982 to Phillips et al., discloses a receiver for a distribution network power line communication system.
When a remote receiver receives a message on the power line, it must demodulate it to obtain the base band data signal contained therein. Although many types of modulation schemes are known to those skilled in the art, one particular type of modulation technique that is applicable to power line communication systems is a phase shift keyed (PSK) system. U.S. Pat. No. 4,379,284, which issued on Apr. 5, 1983 to Boykin, discloses a coherent phase shift keyed (CPSK) demodulator for use with a power line communication system. A distribution network communication system utilizing flexible message routes is disclosed in U.S. Pat. No. 4,427,968, which issued on Jan. 24, 1984 to York. U.S. Pat. No. 4,573,170, which issued on Feb. 25, 1986 to Melvin et al., discloses an apparatus and method for sampling multiple carrier signals when the receiver is used in conjunction with a three phase power line and U.S. Pat. No. 4,467,314, which issued on Aug. 21, 1984 to Weikel et al., discloses an electric utility communication system with a field insulation terminal and a load management terminal.
As more and more electric utility companies utilize the power lines to communicate with remote receivers, certain problems can be experienced. If two power lines which employ these types of communication systems are connected together, transmitted messages can travel along the power lines to destinations for which they are not intended. This can occur when two independent power line communication systems are contained within a single distribution system or when two distribution systems are contained within the same electric utility grid. Experience indicates that power line communication signals can be received from a transmitter that is over 75 miles away from the effected receiver and, theoretically, power line communication signals could travel in excess of 100 miles. Therefore, although there may be various devices connected to power line systems that can effectively block the transmission of these signals, it is apparent that, when a transmitter transmits a power line communication message, many unintended receivers can possibly receive the transmission. This situation presents two distinct problems. First, a receiver could respond to a command that was not intended for its receipt and shed a load at an inappropriate time. Secondly, a receiver can be occupied in the process of receiving and demodulating a message that was not intended for it and thereby ignore another message that is intended for its receipt and response.
Various techniques are presently being utilized to avoid these problems, but they are not infallible in operation. For example, various different frequencies are used by different power line communication systems. Since each receiver is provided with a demodulator that is designed to receive and demodulate a particular frequency, it will ignore messages of different frequencies. However, the number of applicable frequencies is finite and the possibility exists that two independent power line communication systems will utilize the same frequency. The use of individual and block addressing is helpful in preventing a remote receiver from inadvertently following a command which is not intended for its receipt. However, there is a finite limitation on the uniqueness of individual addresses and, in the case of block addresses, it is likely that different electric utility companies will utilize similar numbering schemes to identify its block addresses. Furthermore, even when addressing schemes can be used to avoid the situation in which a remote receiver inadvertently follows a command intended for another receiver, the interference problem described above still exists. A remote receiver can be occupied during the demodulation of a spurious message and thereby miss the receipt of a message intended for its response.
Therefore, it would be helpful to develop a device that prevents the receipt of power line communication messages by receivers for which the messages were not intended regardless of the block address or individual address contained in the message.